Three reactions involving H₂PO₄^– are given below :

                    i) H₃PO₄ + H₂O → H₃O⁺ + H₂PO₄^–

                    ii) H₂PO₄^–+ H₂O → HPO₄^2- + H₃O⁺

                    iii) H₂PO₄^– + OH^–→ H₃PO₄ + O^2-

In which of the following above H₂PO₄^– act as an acid?